1. Field of the Invention
This invention relates to a compensation method and apparatus for a fuel injection amount during engine warm-up of automobiles and the like equipped with electronically-controlled fuel injection devices.
2. Description of Related Art
As one of methods for dealing with unstable combustion using fuel rich mixtures after engine start, the after-start fuel enrichment in the fuel injection quantity amount after engine start is set to be AS1 and AS2, and those AS1 and AS2 are attenuated as time lapses thereafter as shown in FIG. 3D. It must be noted here that AS1 attenuates at high speed. On the other hand, AS2 attenuates at low speed. In addition, there is also a warm-up fuel enrichment compensation WL which is adjusted according to the temperature of the engine coolant as shown in FIG. 3E.
However, these amounts of compensations are set for standard fuels and therefore, in the cases when fuels with different volatilities are used, it may occur that the amount of fuel injection is not properly adjusted to the engine conditions.
For example, crude fuels with higher vaporization points than the standard fuel have bad vaporabilities, and their use, as shown by the solid line in FIG. 3B, results in an overlean condition in air-fuel ratio (A/F) as compared with the standard fuel shown by the dot-and-dash line. Because of this, in spite of the increase in the enrichment amounts of injection after engine start and during engine warm-up, sufficient combustion is not achieved and engine revolution speed NE drops as shown in FIG. 3A, resulting in engine stalls, rough idle and backfire during acceleration. Even with crude fuels, however, if the temperature in the engine combustion chamber and the area surrounding intake valves has increased enough, the fuel's vaporability improves and as a result, engine revolution speed stabilizes and backfire during acceleration does not occur any more.
To deal with the above problem that occurs during the engine warm-up, as disclosed in Japanese Patent Laid-open Publication No. 3-61644, it is proposed that, in the case when the actual revolution speed has fallen excessively below the intended speed, the amount of the fuel injection is increased through fuel enrichment compensation coefficients which correspond with the engine coolant temperature and engine revolution speed.
However, in the above method, the fuel injection amount is increased if the engine revolution speed falls below the intended speed, and such fuel increase ceases when the engine revolution speeds up and reaches the target speed. Thus the air-fuel ratio, which used to be proper, becomes too lean, causing the engine revolution speed to fall and rough idle to occur. Furthermore, because the amount of enrichment is adjusted based on the engine revolution speed as opposed to that, in general, the amount of fuel requirements of engines differ from idle to non-idle conditions, an overlean mixture condition can occur temporarily during acceleration causing poor drivability and backfires.